Since the events of Sep. 11, 2001, the Department of Homeland Security has increased security dramatically in U.S. airports. Such security efforts include screening passengers and carry-on bags and luggage for concealed contraband, such as explosive materials, drugs and/or weapons.
At least some known security scanning systems employ X-ray transmission technology. Although these systems enable the detection of weapons and blades, for example, they lack the capability of efficiently detecting small quantities of explosive materials with a low false alarm rate. Computed tomography (CT) provides a quantitative measure of material characteristics, regardless of location or the superposition of objects, and a substantial advantage over conventional and multi-view X-ray transmission and radioisotope-based imaging systems. In a CT scanner, a large number of precise X-ray “views” are obtained at multiple angles. These views are then used to reconstruct planar or volumetric images. The image is a mapping of the X-ray mass attenuation value for each volume element (or voxel) within the imaged volume.
Systems employing, for example, CT scanners are used widely in airports around the world on checked luggage to detect explosives that pose a threat to aviation safety. These systems employ an X-ray source and opposing detectors that detect X-ray radiation that passes through an object, e.g., a suitcase, as the object is translated along a conveyor.
At least some known scanning systems are capable of detecting most explosive materials and/or other contraband. However, false alarms are occasionally raised due to similarities shared by explosive materials, other contraband and benign materials. Thus, there is a need to reduce the false alarms.
One method employed for clearing alarms includes determining, in addition to the CT number, the effective atomic number of a suspect object. This may be achieved employing a scanner that provides dual energy information. Standard methods include utilization of energy sensitive detectors, dual energy beams, and filtering. Energy-sensitive detectors facilitate collecting data simultaneously at two or more energies. Some of the energy-sensitive detectors are sophisticated and expensive but may be desirable when the throughput is critical and the additional cost is acceptable. The utilization of filters, either at the source or at the detectors, results in small atomic number separation and might not be sufficient for distinguishing the target material from other materials.
Atomic number information may also be collected by irradiating objects at two or more energies. Utilization of dual energy beams is most efficient when the same volume is inspected with two different energies. This reduces the artifacts due to misregistration. One efficient way is to switch the voltage of the x-ray tube very fast in such a way that the gantry moves a very small angle between energies. This requires a very fast-switching x-ray source, which is not readily available at the required timing, voltages and currents used in some CT systems.
An algorithm employed to determine the effective atomic images in CT scanners requires low and high energy projections (sinograms), which are obtained by any of the methods described above. The algorithm consists of employing system model or calibrated parameters that are tuned to match measured projection values for specific basis-materials. The sinograms are then reconstructed to form basis-material images, which are combined to form effective atomic number images. Alternatively, image-based decomposition is applied after the reconstruction.
At least one CT system utilized for checked luggage explosive detection includes a prescan stage to identify a small number of CT scans. The resulting CT scan data is then used to automatically identify explosives or other objects of interest. These scanners operate in the step-and-shoot mode, wherein the slices are collected with the scanned object in a stationary position.